sched,ion: Convert to sched_set_normal()

In an attempt to take away sched_setscheduler() from modules, change
this into sched_set_normal(.nice = 19).

Cc: john.stultz@linaro.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>